In general, Java does not allow operators to be applied to Stringobjects. The one exception to this rule is the +operator, which concatenates two strings, producing a String object as the result. This allows you to chain
together a series of + operations. For example, the following fragment concatenates three strings:
String age = "9";
String s = "He is " + age + " years old.";
System.out.println(s);

This displays the string "He is 9 years old."

One practical use of string concatenation is found when you are
creating very long strings. Instead of letting long strings wrap around within your
source code, you can break them into smaller pieces, using the + to
concatenate them. Here is an example:

// Using concatenation to prevent long lines.
class ConCat {
public static void main(String args[]) {
String longStr = "This could have been " +
"a very long line that would have " +
"wrapped around. But string concatenation " +
"prevents this.";
System.out.println(longStr);
}
}

String Concatenation with Other Data Types

You can concatenate strings with other types of data. For
example, consider this slightly different version of the earlier example:

In this case, age is an int rather than another String,
but the output produced is the same as before. This is because the int value in age is
automatically converted into its string representation within a String object. This string
is then concatenated as before. The compiler will convert an operand to its string equivalent
whenever the other operand of the + is an instance of String.

Be careful when you mix other types of operations with string
concatenation expressions, however. You might get surprising results. Consider the
following:

String s = "four: " + 2 + 2;
System.out.println(s);

This fragment displays

four: 22
rather than the
four: 4

that you probably expected. Here's why. Operator precedence
causes the concatenation of "four" with the string equivalent of 2 to take
place first. This result is then concatenated with the string equivalent of 2 a second time. To complete the
integer addition first, you must use parentheses, like this: